provenance, and derivation from the Rio de la Plata craton is highly unlikely. considerations, previously zircon data, palaeocurrent and structural data, suggest that these rocks must have had an origin within Gondwana-forming blocks, for which the closest ide ntifiable somces are 'Brazilian' and 'African' (Namaqua Natal). Consequently, the model for the orogeny is that the Rio de la which was at the time attached to the large Amazonia craton and other smaller continental blocks, such as Arequipa Antofalla and Rio Protracted relative displacement of the RPC after the Orogeny 1ed to its fina1 !-'U.>lU'UH.
A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Córdoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513 ± 31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos-Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490 ± 5 Ma. Younger monzogranites gave Rb-Sr whole-rock ages of 470–450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial
87
Sr/
86
Sr (0.7075–0.7105) and low ɛNd
t
(−4.6 to −6.3), inherited from lower crust. Sm-Nd model ages of 1600–1700 Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.
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